Well, no, obviously it's a feature of Distributed Version Control Systems in general.
In fact, that's probably why OP wrote "This is why I love git (and distributed version control systems in general)" and didn't in any way claim it was a feature of GitHub?
And do you have some kind of reference for git being less memory-efficient (are you talking RAM-wise?) vs Mercurial or TFS? On its face its pretty hard to believe; git is pretty light-weight.
I'm certainly not slamming Git for working exactly the way it was intended.
As an example when you clone a repository, it downloaded it, then in goes through a process of 'Resolving Deltas'. This is it going through the history applying the changes/deltas to the files so that they are up to the most recent revision.
The only files that are added in full are binary files.
Some reading: http://git-scm.com/book/en/Git-Internals-Packfiles :)
So, git can be quite heavy on disk usage, until all the loose objects are packed.
Also, maybe don't confuse git with GitHub, as your original comment did.
The only SCM I like less than TFS is SourceDepot. The bane of my existence. It's sad when I can rant about it abstractly and my father can sympathize.